A hot-rolled steel sheet having a tensile strength (TS) of 440 MPa grade or lower has been used for members of passenger cars and trucks such as frames and chassis. However, recently, since there has been a trend toward strengthening and thickness reduction of automotive steel sheets in order to increase the collision resistance of automobiles and to protect the global environment, investigations have been started regarding the use of a high strength hot-rolled steel sheet having a TS of 590 MPa grade, a TS of 780 MPa grade or a TS of 980 MPa grade or higher.
Since many automotive members have complicated shapes which are produced by performing press forming, it is necessary that the steel sheets for the members have not only high strength but also excellent formability. On the other hand, in the light of ensuring that automobile bodies have sufficient corrosion resistance while the thickness of the steel sheet is decreased, there is a demand for a coated steel sheet which is manufactured by giving corrosion resistance to a steel sheet, in particular a galvannealed steel sheet which is excellent in terms of corrosion resistance and weldability and can be manufactured at low cost.
In the past, there have been some examples of high strength hot-rolled steel sheets or high strength galvanized steel sheets having excellent formability and methods for manufacturing these steel sheets. For example, Patent Literature 1 discloses a high strength steel sheet having a TS of 590 MPa or more and excellent formability and a method for manufacturing the steel sheet, the method including smelting a steel having a chemical composition consisting of, by mass %, C: 0.02% to 0.06%, Si: 0.3% or less, Mn: 0.5% to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less, N: 0.006% or less, Mo: 0.05% to 0.5%, Ti: 0.03% to 0.14% and the balance substantively being Fe, and hot-rolling the smelted steel under the conditions that the finishing rolling temperature is 880° C. or higher and the coiling temperature is 570° C. or higher so that the steel sheet has a microstructure substantively being a ferrite single phase in which carbides containing Ti and Mo and having an average grain diameter of less than 10 nm are dispersed.
In addition, Patent Literature 2 discloses a method for manufacturing a galvanized high strength hot-rolled steel sheet, the method including smelting a steel having a chemical composition comprising, by mass %, C: 0.01% to 0.1%, Si: 0.3% or less, Mn: 0.2% to 2.0%, P: 0.04% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.006% or less, Ti: 0.03% to 0.2%, one or more of Mo: 0.5% or less and W: 1.0% or less and the balance being Fe and inevitable impurities, hot-rolling the smelted steel in a temperature range in which an austenite single phase is formed, coiling the hot-rolled steel sheet at a coiling temperature of 550° C. or higher in order to manufacture a hot-rolled steel sheet having a microstructure being a ferrite single phase, performing descaling on the steel sheet and directly performing galvanizing on the descaled steel sheet so that the galvanized steel sheet has a chemical composition in which, by mass %, the relationship 4.8C+4.2Si+0.4Mn+2Ti≦2.5 is satisfied and a microstructure including, in terms of area ratio, 98% or more of a ferrite phase in which precipitates containing one or more of Ti, Mo and W and having a size of less than 10 nm are dispersed, and in terms of atomic ratio, the relationship (Mo+W)/(Ti+Mo+W)0.2 is satisfied.
However, in Patent Literatures 1 and 2, since fine carbides containing, for example, Ti and Mo are precipitated in a ferrite phase, it is necessary to perform coiling at a coiling temperature (hereinafter, also referred to as CT) of 550° C. or higher after finishing rolling has been performed. In the case where coiling is performed on a hot-rolled steel sheet containing chemical elements such as Si and Mn which have greater affinities for oxygen than Fe (hereinafter, also referred to as easily oxidized chemical elements) at such a high CT, internal oxides containing easily oxidized chemical elements are formed in the surface portion of the hot-rolled steel sheet. Therefore, since a Zn—Fe alloying reaction excessively progresses in subsequent galvanizing and alloying treatments, there is a problem in that there is a decrease in coating adhesiveness. Moreover, in the case where a large amount of internal oxides are present in the surface portion of the base steel sheet, the internal oxides become the origins of fine cracks which occur in the surface portion of the base steel sheet and in a zinc-coated layer when stretch flange forming is performed. Therefore, there is a problem in that there is a decrease in corrosion resistance of a portion subjected to stretch flange forming.
On the other hand, in the case where coiling is performed at a lower CT in order to prevent internal oxides from being formed, since there is an insufficient amount of carbides precipitated, and since a microstructure such as a pearlite phase grows, there is a decrease in strength and formability. In addition to that, when the hot-rolled steel sheet is subsequently annealed in a continuous galvanizing line, since Ti which is present in the non-precipitated solid solute state is concentrated in the form of oxides on the surface of the base steel sheet, small coating defects and non-uniform alloying unevenness occur after a galvanizing treatment or an alloying treatment has subsequently been performed, which results in a significant deterioration of the surface appearance.